After (lots) of fiddling and trial and error, which included referencing my Note 5 gps.conf and flashing xiaomi.eu and mokee ROM on this poor Mi4, I’ve got something that works, eventually (first time the fix took 510 seconds, now even more).
Of course, now time to test how it behaves with Battery Optimization turned off for OruxMaps and runner apps.
In hopes it helps someone in the future, here’s the gist file